Support MPU9250 over I2C

[bryanjhv]

What kind of new feature are you looking for?

• Hardware: Support MPU9250 over I2C, currently only implementation uses SPI which is fine but many mixed chips like the GY-91 expose it as I2C. Another option would be to make mpu9250.Proto interface with public methods so one can provide an implementation for it (like SpiTransport does).

Do you plan to:

• Contribute an initial driver: No
• Write unit tests: No
• Update https://github.com/periph/cmd to use the new functionality: No

[bezineb5]

I have an implementation of MPU9250 over I2C, with embedded AK8963 I2C magnetometer. However, it's not based upon the code of the existing MPU9250 driver, so I never submitted it (the interfaces are not compatible).
I can share the code if still needed.
@maruel Any interest in this MPU9250 I2C + AK8963? (the AK8963 is actually the magnetometer part of the MPU9250, exposed as a separate device on the I2C bus)

[maruel]

Sure!